Due to the rapid increase in the number of elderly people in the population, bone disorders such as osteoporosis and osteomalacia are becoming more common. Osteoporosis generally refers to a disorder wherein the amount of bone mineral in the skeleton decreases, and the weakened bone is more susceptible to fracture. Osteomalacia is a softening of the bones generally caused by a deficiency of vitamin D. Fractures of the spine (crush fractures) and of the radius (Colles fractures) or femoral neck can decrease the quality of life or shorten the life of the increasing numbers of elderly in the population. However, there are steps which can be taken to reduce the probability of such injuries if the individual is known to be at risk. Consequently, an accurate, cost-effective method for evaluating an individual's susceptibility to bone fracture is needed.
In present methods of assessment, bone mineral content or bone mineral density are used to estimate the fracture risk from osteoporosis. Such methods have the disadvantage that they do not completely describe the mechanical strength changes that occur in the skeleton with age and therefore only approximately estimate the increased risk of fracture in an aging individual.
The typical method for determining bone mineral content employs photon absorptiometry. In this method, a beam of x-rays is passed through the bone being measured, either in a rectilinear fashion using a pencil beam of x-rays, or in a scan with a fan-like beam of x-rays. The attenuation of the x-ray beam is used to calculate the amount of bone mineral present through the use of the attenuation formula for x-rays: EQU I=I.sub.o xe.sup.-.mu..sbsp.x.sup.x ( 1)
where I=attenuated x-ray beam
I.sub.o =unattenuated x-ray beam PA2 .mu.=lineal attenuation coefficient of absorber PA2 x=thickness of absorber
Bone strength, however, does not depend only on bone mineral content. Bone mineral content is certainly a major factor in determining bone strength, but strength is also affected by the mechanical structure of the bone. In a simple sense, if the long bones of the body were thought of as pieces of tubing, the bone mineral content would relate to the total amount of material in the tubing per unit length. However, the mechanical strength of such a tubing would also be significantly affected by the tubing diameter and tubing thickness. These diameter and thickness factors are not accounted for in a simple measurement of bone mineral content.
Thus, there is an existent and continuing need for a better method of assessing bone strength, especially a method which takes into account not only the total amount of material present (bone mineral content), but also the geometrical distribution of the material present.